Novel foaming process for production of foam materials

Abstract

A production process can produce foam materials from polymer compositions. This process involves preheating in the foaming of polymers containing blowing agents and subsequent foaming by a thermal process assisted by microwaves.

Claims

1. A process for producing foams, comprising: preheating polymer compositions to be foamed, and foaming the polymer compositions containing a blowing agent in an apparatus having a thermal heating device for heating to an internal temperature of the apparatus T.sub.S and irradiating simultaneously the polymer compositions by at least one microwave source of the apparatus.

2. The process according to claim 1, wherein the polymer compositions are preheated in the apparatus at a temperature between a minimum of 10° C. below and a maximum of 10° C. above the internal temperature of the apparatus T.sub.S.

3. The process according to claim 1, wherein a temperature differential between a coldest point and a hottest point within the polymer compositions after the preheating is not greater than 15° C.

4. The process according to claim 1, wherein the preheated polymer compositions are compositions are supplied directly to the foaming process in the apparatus.

5. The process according to claim 1, wherein the polymer compositions are a material that forms a rigid foam after the foaming.

6. The process according claim 1, wherein polymer compositions, prior to the foaming, are preheated to a preheating temperature T.sub.v above a glass transition temperature T.sub.g of the polymer compositions.

7. The process according to claim 1, wherein a preheating temperature T.sub.v is between 110 and 190° C., and a preheating time is at least 60 min.

8. The process according to claim 1, wherein microwave radiation from the at least one microwave source has a frequency between 0.85 and 6.0 GHz.

9. The process according to claim 1, wherein the polymer compositions comprise PMI, wherein a preheating temperature T.sub.V is between 120 and 190° C., and wherein an internal temperature of the apparatus T.sub.S is between 180 and 240° C.

10. The process according to claim 1, wherein the polymer compositions comprise PMMA or methacrylate-based copolymer, wherein a preheating temperature T.sub.V is between 110 and 140° C., and in that the wherein an internal temperature of the apparatus T.sub.S is between 120 and 190° C.

11. The process according to claim 5, wherein the rigid foam is selected from the group selecting of P(M)I (poly(meth)acrylamide), PMMA, a methacrylate-based copolymer, polyvinyl chloride (PVC), polypropylene (PP), polyurethane (PU), a polysulfone, and a poly(ether)imide.

12. The process according to claim 7, wherein the preheating time is at least 100 min.

Description

PROBLEM

[0007] The problem addressed was that of developing an economically viable process for foaming (rigid) foams that simultaneously leads to a product having very homogeneous pore structure. Particularly for the production of rigid foam blocks, such as PMI- and PMMA-based foams that cannot be produced by extrusion processes, for example, on account of their high molar masses of the matrix polymer, an economically viable foaming process was to be developed. A further aim was to distinctly shorten the process for production of rigid foam sheets and hence to make the foaming process more economically viable.

[0008] Further problems not discussed explicitly at this point may be apparent hereinafter from the prior art, the description, the claims or working examples.

SOLUTION

[0009] The objects are achieved by a novel process for producing foams, characterized in that, in this process, polymer compositions containing a blowing agent are foamed in an apparatus, and in that the polymer compositions to be foamed are preheated prior to the foaming. Said apparatus has a thermal heating device for heating to an internal temperature of the apparatus T.sub.S and at least one microwave source by means of which the polymer compositions are irradiated simultaneously.

[0010] The polymer composition is preferably a material that forms a rigid foam after foaming. This is more preferably P(M)I (poly(meth)acrylamide), PMMA, a methacrylate-based copolymer, PVC (polyvinyl chloride), PP (polypropylene), PU (a polyurethane), especially highly crosslinked PU, a polysulfone or a poly(ether)imide.

[0011] In the preheating step, the polymers to be foamed, prior to the foaming conducted in the apparatus, are preheated at a temperature T.sub.v between a minimum of 80° C. below and a maximum of 10° C. above the internal temperature of the apparatus T.sub.S. Preferably, T.sub.v is at a minimum of 60° C., more preferably a minimum of 40° C., especially a minimum of 20° C., below the internal temperature of the apparatus. Also preferably, T.sub.v is not higher than T.sub.S, and is more preferably at least 10° C. lower than T.sub.S.

[0012] The preheating and the actual foaming can be effected in the same apparatus, for example in the form of an optional temperature change and a switch-on of the microwave sources after the preheating. However, preference is given to conducting the two steps in separate apparatuses. For instance, the preheating can be effected simultaneously in a large oven with multiple workpieces. Individual workpieces are then taken therefrom for the foaming.

[0013] It has been found to be particularly advantageous to conduct the preheating in such a way that the temperature differential thereafter between the coldest and hottest point within the polymer compositions after preheating is not greater than 15° C., preferably not greater than 10° C. Equally preferably, the preheated polymer composition is then sent directly to the foaming in the apparatus.

[0014] It is especially advantageous to preheat the polymer composition, prior to the foaming, to a preheating temperature T.sub.v above the glass transition temperature T.sub.g of the polymer composition. Ideally, the polymer composition is elastic when the actual foaming is commenced.

[0015] For many materials, even those with a relatively high crystalline component, for example PP, in a simplified manner, a preheating temperature T.sub.v between 110 and 190° C. has been found to be favourable. In general, the preheating time is at least 60 min, preferably at least 100 min.

[0016] Preference is given to using microwave radiation having a frequency between 0.85 and 6.0 GHz. It has been found to be favourable to use polymer sheets having a thickness between 10 and 30 mm, preferably between 20 and 25 mm, as polymer composition.

[0017] The actual foaming can be effected, for example, within 2 to 30 min, preferably within 5 to 20 min, in the apparatus.

[0018] In a preferred embodiment of the present invention, the polymer composition is PMI. For this embodiment, the preheating temperature T.sub.v is preferably between 120 and 190° C. and the internal temperature of the apparatus T.sub.S between 180 and 240° C.

[0019] In another preferred embodiment of the present invention, the polymer composition is PMMA or a methacrylate-based copolymer. For this embodiment, the preheating temperature T.sub.v is preferably between 110 and 140° C. and the internal temperature of the apparatus T.sub.S between 120 and 190° C.

[0020] A major advantage of the process according to the invention is that it can be conducted in an environmentally benign manner and in very short cycle times and simultaneously in a material-conserving manner. In particular, by the process of the invention, surprisingly uniform pore sizes and pore size distributions are obtained throughout the foam part.

[0021] A further processible rigid foam is PVC foam. This rigid foam is universally known from fibre composite technology and sandwich production for wagon construction and the production of wind turbines and from boatbuilding. The finished foam sheets can be processed analogously to the PMI foam sheets.

[0022] The same applies to rigid PP foams. PP foams are especially known as insulation material, in transport containers and as sandwich material. PP foams can comprise fillers and are available commercially, mostly in a density range from 20 to 200 kg/m.sup.3.

[0023] Features of rigid PU foams, in comparison with flexible PU foams, are in turn a more closed pore structure and a higher degree of crosslinking. Rigid PU foams can also comprise relatively large amounts of inorganic filler materials.

[0024] The density of the rigid foam material can be selected relatively freely. Foams may be used, for example, in a density range from 25 to 220 kg/m.sup.3.

[0025] In principle, the workpieces of the invention made from a rigid foam are very widely usable.